Electromagnetic compatibility can be an important issue in electronic devices, for example to comply with international standards to minimize Electromagnetic Interference (EMI). Switch-mode power supplies may be designed to conform to these standards to reduce the transient noise on the power line. In a power supply with two power lines and a protective earth, the transient noise may include differential noise and common mode noise. Common mode noise may be expressed as a potential difference with respect to a protective earth (PE). The differential mode noise may be seen as an injected potential difference between the live and neutral mains supply.